1. Field of the Invention
The present invention relates to an electronic apparatus driven by, for example, a battery power and to a power supplying method, in particular, to a structure and a method that allow a power consumption to be reduced.
2. Description of the Related Art
Portable apparatuses such as a cellular phone, a PDA (Personal Digital Assistance), and a portable computer normally use a battery as a power supply. A battery drive type system is composed of, for example, a system LSI (Large Scale Integrated Circuit), a battery, a power control IC (Integrated Circuit), and so forth. A system LSI is an LSI of which a CPU, a memory, and various peripheral functional blocks are integrated into one chip. In recent years, a portable apparatus has not only a communication function as a cellular phone function, but also a multimedia communication function, a processing function, and so forth. To accomplish such portable apparatuses, system LSIs have been finely structured, for example, on the order of 0.3 μm or less.
Since the standby time of a battery drive type portable electronic device such as a cellular phone depends on the power consumption thereof, it is important to reduce it. For example, as a related art reference, Japanese Patent Laid-Open Publication No. 2002-170933 discloses a method for reducing a power consumption in such a manner that while the power of a system LSI is always turned on, the supply of a clock is stopped and the system LSI is placed in a standby state so as to reduce the power consumption.
In the method disclosed in the related art reference, since the power of the system LSI is always turned on, a sub threshold leak current flows in transistors of a system LSI (for example, a MOS LSI), when the LSI is in the standby state, the power consumption thereof adversely increases. A sub threshold leak current is a current that flows from a high power supply voltage to a low power supply voltage even if a MOS transistor does not operate.
As a countermeasure for decreasing a sub threshold leak current in an MOS LSI, threshold voltages of transistors are raised. The threshold voltages can be raised by performing process tuning or supplying a substrate potential. However, when a device is normally operated, if threshold voltages of transistors are raised, the speed of the device lowers. Thus, such a method is not suitable for applications that operate at high speed.
As another method for suppressing a sub threshold leak current, a power supply voltage could be lowered. Likewise, from a view point of high speed operation, this method is not suitable. In addition, it is impossible to perfectly shut off a current. Thus, the operation speed and the power consumption, which are typical indexes that represent the performance of an LSI, are always reciprocal factors. Thus, it is very difficult to improve the performances of both the factors at the same time.
As another method, a power control IC and a system LSI might be separately structured. In this case, the power control IC generates a power and a power on/off control signal. The power control IC supplies the power and the power on/off control signal to the system LSI so that when the system LSI is in the standby state, the power is turned off. In such a structure, the power consumption might be reduced in the standby state. Normally, a power control IC is composed of an analog IC circuit such as a regulator. Thus, when a complicated digital circuit that generates a control signal is built in a power control IC, since it is necessary to newly design a power control IC, the designing work thereof will be required. As a result, the cost of the power control IC will rise.